For the production of board materials, such as particleboards, curable formaldehyde based glues are generally used and these glues are cured by means of acids or acid forming compounds. Urea-formaldehyde resins are mainly used for the gluing, but melamine-formaldehyde resins, mixtures of urea and melamine resins, mixed condensates of urea, melamine and formaldehyde as well as mixed condensates containing phenolic compounds can also be used. From all these glues formaldehyde can be released at the manufacturing of the products or at the subsequent use of these.
It is well known that formaldehyde is irritating and causes allergy and it is thus, with respect to occupational and environmental hygiene, desired to reduce the amount of free formaldehyde to the greatest possible extent and also desired to absorb the formaldehyde which can be emitted from the cured glue. A number of different methods for reducing the content of free formaldehyde have been suggested, e.g. addition of formaldehyde absorbing substances to the glue compositions or to the cellulose material at the board production, post-treatment of the finished products, use of glues with a low mole ratio.
The last mentioned method is in particular true about the use of urea-formaldehyde resins, whereas previously resins with a molar ratio of urea to formaldehyde substantially within the range of from 1:1.4 to 1:1.8 were used, nowadays resins with a molar ratio about or below 1:1.4 are used instead to a great extent. Despite the reduced mole ratio certain amounts of free formaldehyde will still exist and it is not possible to lower the mole ratio too far to avoid the problems of free formaldehyde, as other properties, particularly the tensile properties of the board materials, would then be negatively influenced. Low mole ratios do also impair the storability of the resin, the dilutability etc.
Additions of urea or other formaldehyde absorbing agents to the resin before the use thereof have also been employed, but also this method does of course lead to a lowered mole ratio with the consequence that the strength properties of the produced boards are impaired. Further, addition of formaldehyde absorbing agents to the particles before, during or following the gluing of these has been employed. If, for example, particles that have not been glued are sprayed with a formaldehyde absorbing agent at such conditions that substantial amounts of the agent will be present on the surfaces of the particles this will, in a manner corresponding to that of addition to a glue composition, have a detrimental effect on the glue which is used and thereby also on the properties of the manufactured products.